This invention relates to corrections to color separations for color printing and particularly to modifying signals representative of various colors in a picture, specifically one in which the colors do not vary with time, to reproduce a color picture that is a true representation of the original picture.
Reproducing color images by printing with inks requires adjustments to the signals that control the inks. Most systems printing color images use a three color, or tristimulus, basis. The signals originate from scanning a color image to be reproduced and separating the tristimulus color components. In cases where the original image is a color photograph, it is desirable that the reproduction be an accurate reproduction of the original. The hue versus saturation plots of color inks and color photographs are not the same and both are different from the response of the human eye. In addition, nonlinear errors are introduced by the scanning system.
Further complications are introduced because the scanning system responds to additive color mixtures whereas the perception of color images printed on a medium responds to subtractive color mixtures. The relations among the signals can be represented by equations attributed Neugebauer, but reproducing color images accurately have required the intervention of an operator to make the final adjustments. This has been done by making printing plates and test impressions from which adjustments are made. Modern systems use color television monitors which makes the adjustments more convenient, faster, and cheaper. Still, the systems use operator intervention exercising subjective, albeit it experienced, judgements.
The prior art systems for color correction in color reproduction require the intervention of an operator to adjust the signals controlling the inks based on subjective judgments of the results. Usually test reproductions are required on which the operator bases the judgment of changes required.
U.S. Pat. No. 4,500,919 describes a system for reproducing a color original by extracting three color components which are corrected by an operator manipulating a television image to introduce aesthetic corrections. This requires no knowledge of the ink and media characteristics.
U.S. Pat. No. 4,590,515 reduces CMY inks and increases black to compensate for the reduction. This prevents color casts occurring in the reproduction of gray tones. The adjustments for color fidelity are not automatic.
U.S. Pat. No. 4,841,360 electronically enhances images and then adjusted to the characteristics of the copy material. The image is then modified proportional to the illumination intensity of the displaying CRT followed by correction to adjust gray balance (contrast). The image printed via the CRT with adjustments to the nonlinear characteristics function (line) of the CRT by a suitable amplifier so that the CRT does not distort the gray balance. There is no teaching of automatically correcting for nonlinearities in color printing.
U.S. Pat. No. 4,845,550 adjusts the coefficients of a polynomial representing color correction using six hue signals. The procedure equalizes the weights of the three color signals processed so that the six hue signals produced therefrom divide a color space. Nothing is shown for automatically correcting three color printing.
The invention is useful in color printing because it is automatic, therefore faster and less susceptible to subjective judgements. It also precludes the necessity of printing proof sheets. Furthermore, it corrects the input color separation space so that the input values from different scanning systems can be printed together. The corrections to the printing color space are straightforwardly computational which allows the conversions to be made efficiently on systems not readily adaptive to other methods of color conversion such as table lookups in interpretive systems.
In accordance with the invention, a method of processing color component signals for accurately reproducing color images includes scanning the color image to be reproduced to derive color components of the image. These components may be the tristimulus values of red, green, and blue, viz., the RGB components. Errors introduced by the scanning process are corrected, preferably using look up tables based on reference signals derived from the signals resulting from scanning test targets on the media upon which the reproduction is to be made.
Irregularities in the reproduced image introduced by the characteristics of the ink used for reproduction are corrected by deriving parameters of the individual inks used and applying nonlinear modifications to the signals without the need for operator intervention.